In conventional X-ray imaging systems, the object to be imaged (typically a patient) is placed between an X-ray source and a plate of photographic film. The interaction between the patient and the X-ray radiation is captured and stored on the film. The film may then be viewed for diagnostic purposes via an apparatus which back-lights the film. Although diagnostic images on film have good resolution and contrast characteristics, there are significant disadvantages to using such media for storing images. First, physical examinations call for a predetermined level of X-ray energy depending upon which part of the body is being X-rayed, i.e., a peak voltage and maximum milliampere level for powering the X-ray source, and in the case of a pulsed X-ray source, the time duration of each pulse driving the pulsed X-ray source. If the parameters of the X-ray exposure are wrong the resulting X-ray image recorded on film tends to be either under or over exposed, because of the mismatch between the X-ray flux and the film dynamic range, resulting in poor diagnostic quality images.
In the U.S. the estimate of X-ray images recorded on film that are so poor as to require the retaking of the image ("recall") is estimated to be around 20%. Radiologists expect this number to go up, because of greater reliance on X-ray technicians who are typically less trained and often less skillful than the average radiologist.
Digital X-ray images, in contrast to conventional analog images stored on film, can be archived and stored in picture, archive, communication systems (PACS). The use of PACS facilitates transmission of images via networks and modems to support remote diagnosis. The data is therefore easily stored in memory and archived, and more readily transferred than information contained in X-ray film, without deterioration of data. Thus, various digital processing algorithms and techniques become available making it easier to process the data, such as spatial filtering and other image enhancement techniques and transfer the data from one location to another, by network or modem, for example.
In recent years techniques have been developed to overcome these disadvantages of X-ray film. A number of extant methods and associated technologies have been developed, some designed to provide digitized systems, eliminate film, or otherwise improve on the use of film. The following is a summary of the strengths and weaknesses of the major technologies in use today.
Radiographic Equalization: Radiographic equalization is a process in which the entrance X-ray exposure to a patient is increased locally in areas of high attenuation and decreased over areas of low attenuation so as to provide a more homogeneous distribution of X-ray exposure to the recording medium. Radiographic equalization techniques include scanning equalization radiography, scanning slit equalization radiography, digital beam attenuation, light equalization radiography, and rotary-scanning equalization radiography. The latter includes the steps of scanning an object with an x-ray fan beam by traversing the object with the fan beam at each of a plurality of scan angles as defined by the plane of the fan beam. Examples of the latter are described in:
Sabol et al., "Practical Application of a Scan-Rotate Equalization Geometry to Mammography," Medical Physics, 23(12), December 1996. PA1 Sabol et al., "Analytical Description of the High and Low Contrast Behavior of a Scan-rotate Geometry for Equalization Mammography," Medical Physics, 23(6), June 1996. PA1 Sabol et al., "Rotary Scanning Equalization Radiography: an Efficient Geometry for Equalization Mammography," Medical Physics, 21(10), October 1994. PA1 Boone et al., "Filter Wheel Equalization in DSA: Simulation Results," Medical Physics, 20(2), March/April 1993.
Rotary scanning equalization radiography is analogous to first generation computed tomography, as described Sabol et al., "Analytical Description of the High and Low Contrast Behavior of a Scan-rotate Geometry for Equalization Mammography," Medical Physics, 23(6), June 1996.
Film Digitizers: The output of a conventional imaging system (i.e., film) may be digitized with a scanning device, so that the costs associated with the storage and retrieval of film are eliminated. The disadvantages of this method are that the use and associated cost of film, along with the disadvantages noted above, have not been eliminated; a relatively high cost, high quality scanning device must be procured and maintained; and additional labor costs are incurred to digitize the films. An exemplary medium resolution scanning device is the ScanJet 6100C, manufactured by Hewlett Packard of Palo Alto, Calif. The ScanJet 6100C has a specified resolution of 600 dpi with up to 2400 dpi of enhanced resolution. An exemplary high resolution scanning device is the Scanmate 5000, manufactured by ScanView Incorporated of Foster City, Calif. The Scanmate 5000 has a specified resolution of 5000 dpi.
Photo Stimulated Luminescence (PSL): The film/screen combination used in a conventional X-ray system may be replaced with a plate that stores X-ray flux. The stored flux is read out at a later time by stimulating the material with a laser and measuring emitted light. Digital images are formed by rectilinearly scanning individualized pixelated areas of the plates and digitizing the emitted light. An advantage of this system is that film is completely eliminated. Another advantage is that the signals representative of pixels of a latent image on a plate can be digitized over a sufficiently large dynamic range so that one should not have to retake the X-ray image. In addition, at least with certain substrate materials, the X-ray dosage can be reduced because the material used has a higher detection quantum efficiency (DQE) than that of standard X-ray film. Thus, even if recalls are required, the amount of X-ray exposure experienced by the patient is comparatively reduced. In addition, the substrate is typically erasable and reusable, while X-ray film is not.
The current disadvantage of this PSL system is the cost of the read out system and the labor costs incurred to process the storage material. An exemplary PSL system and method is described in U.S. Pat. No. 4,527,060, entitled RADIATION IMAGE READ-OUT METHOD AND APPARATUS, invented by Ishikawa et al., assigned to Fuji Photo Film Company, Ltd., of Japan. Fuji Photo Film Company is one of the major suppliers of PSL systems.
Image Intensifier Tubes (IIT). The film/screen combination used in a conventional X-ray system may be replaced with an IIT. An IIT comprises four primary components: a photo cathode, a microchannel plate, a screen and a housing which holds the other components stationary relative to one another. As X-ray photons which have propagated through a patient strike the photo cathode (essentially a flourescent screen), electrons are emitted into the interior of the tube. The electrons are accelerated to produce a small image on a microchannel plate, which is a second fluorescent screen. The image on the second screen is captured using a video camera with a digital output, such as a CCD camera. The resulting system receives an X-ray beam and produces a direct digital output. However, the spatial resolution and dynamic range of an IIT based system are typically less than conventional film. IIT-based systems are commonly found in cardiac scanners. An exemplary IIT is described in U.S. Pat. No. 3,708,673, entitled IMAGE INTENSIFIER TUBE, invented by Blacker et al., and assigned to The Machlett Laboratories, Inc., of Springdale, Conn.
Semiconductor Detectors: The film/screen combination used in a conventional X-ray system may be replaced with a two dimensional array of semiconductors. X-ray photons which have propagated through a patient are either directly detected in the semiconductors or are indirectly detected by placing a scintillating material on top of the array. In the former case, the semiconductor reacts directly to the X-ray energy. In the latter case, the scintillating material converts the X-ray energy into light which is within the detection range of the semiconductor, and the semiconductor reacts to the light. Each element in the array detects X-rays in an area which is approximately 100-microns by 100 microns. The energy detected by each element of the array is digitized and then saved as a pixel in a final digital image. To replace film in general radiographic practice, the array needs to have on the order of 2000 by 2000 elements. The advantage of this system is that film is completely eliminated. The disadvantage of this system is the cost of the detectors and their associated electronics. An exemplary semiconductor detector array is described in U.S. Pat. No. 5,079,426, entitled MULTI-ELEMENT-AMORPHOUS-SILICON-DETECTOR-ARRAY FOR REAL-TIME IMAGING AND DOSIMETRY OF MEGAVOLTAGE PHOTONS AND DIAGNOSTIC X RAYS, invented by Antonuk et al., assigned to The Regents of the University of Michigan.
Outside of cost, the digital X-ray imaging systems employing semiconductor detectors currently are the best for film replacement. Their dynamic range and spatial resolution are comparable to, if not better than, conventional film, unlike IIT systems. Furthermore, the output of the detector can be fed directly into a PACS, unlike systems using PSL and film digitizers. There is thus a need for a less costly digital X-ray imaging system having spatial resolution and dynamic range sufficient for medical diagnoses.
It is an object of the present invention to substantially overcome the above-identified drawbacks of the prior art.
It is a further object of this invention to provide an X-ray detection system to replace conventional photographic film imaging systems.
It is another object of this invention to provide an X-ray detection system which provides a direct digital output.
It is a further object of this invention to provide an X-ray detection system which provides a direct digital output having a spatial resolution and dynamic range sufficient for medical diagnoses.